1. Field of the Invention
The present invention relates to a process for the production in continuous of phthalodinitrile. More particularly, the present invention relates to a process for the production in continuous of isophthalodinitrile or terephthalodinitrile by reaction of the respective bis(trichloromethyl)benzenes with ammonia in stream phase on a fixed bed of a dehydration catalyst. Optionally this reaction is carried out in the presence of aqueous steam.
2. Prior Art
Dinitriles of the acrylic acids can only be prepared by complex chemical methods or adopting sophisticated and expensive equipment. The classical method, from carboxylic acids, by reaction with ammonia in steam phase on dehydration catalysts, is often not easily applicable due to the chemical-phisical characteristics of the starting acids. In fact many solid aromatic acids, such as for example, phthalic acids, are difficult to vaporize, melt at high temperatures and already at these temperatures they begin to decompose developing carbon dioxide.
Consequently, attempts have been made to overcome these difficulties in different ways, for example dispersing the pulverized aromatic acid in a stream of inert gas at a temperature below its melting point, mixing it with a very hot ammonia stream, and conveying the vaporized reaction products on a dehydration bed. This method, claimed in U.S. Pat. No. 3,070,621, which avoids the melting of the aromatic acid, considerably reduces the decomposition phenomena and therefore the forming of deposits and crusts on the reactor walls, especially in that zone used as vaporizer. Anyway it is evident that this results considerable installation difficulties and it is therefore clear why successively other ways were tried, such as for instance the use of phthalic acid derivates, which vaporize more easily: for example, ammonium salts, diamides and above all esters of such acids, wherein the alkyl chain contains from 1 to 4 carbon atoms. Thus, for example, DTAS 1,279,020 claims the preparation of aromatic nitriles from methyl esters of the correspondent acids: these processes, if on the one hand simplify the installations, on the other hand raise the new problem of the precursor production, which precursors are not easily available on commercial and industrial scale.
Only in the last 20 years new processes of ammono-oxidation were industrially developed, which consist in reacting mixtures of aromatic hydrocarbon containing an alkyl chain, oxygen or air, and ammonia at high temperatures on suitable vanadium, tungsten etc. catalysts. However, these processes require high technologies, suited to effect the reactions in steam phase, on fluidized beds; separation and recycle of the unreacted compounds or of the reaction intermedies, the use of particular and sophisticated catalysts, and in addition plants of considerable size are needed, which require high capital investements.
The Applicant has claimed in its patent application No. 20077 A/86 filed on Apr. 14, 1986 a process for the production in continuous of isophthalodinitrile and terephthalodinitrile by amidation and simultaneous dehydration of the respective acid chlorides in steam phase on a fixed bed of a dehydration catalyst.
Chlorides of phthalic acids are commercially available in industrial quantities or can be obtained according to different kinds of synthesis. For instance a classical way is the reaction of the corresponding aromatic acids with chlorinating agents such as thionyl chloride, phosphorous oxychloride, phosgene etc. A more convenient synthesis involves the photochlorination of the respective xylenes and the subsequent partial hydrolysis of the resultant .alpha.,.alpha.,.alpha.,.alpha.',.alpha.', .alpha.', hexachloroxylenes by means of H.sub.2 O or CH.sub.3 OH (see U.S. Pat. No. 3,835,187) or the melting with equimolecular quantities of the corresponding aromatic acids (see British Pat. No. 946,491).